Download a force

A100 Movie Special
Tuesday, March 23
Swain West 119
7:00 pm (153 minutes)
Winner of several awards
20 activity points!
BYOP
(Bring Your Own Popcorn)
Matter, Forces and Motion
Scalars and Vectors
Scalar: a quantity described solely by its
size (and units)
Vector: a quantity described by its size
AND direction
• speed – rate at which an
object moves [e.g., m/s].
A scalar quantity.
• velocity – an object’s speed
AND direction, [e.g.,10 m/s
east]. A vector
quantity.
• acceleration – a change in an
object’s velocity, i.e., a
change in speed OR direction
[m/s2]. A vector quantity.
Momentum (p) – the mass of an object times its
velocity (p=mv)
Force (f) – anything that can cause a change in an
object’s momentum
As long as the object’s mass does not change, a force
causes a change in velocity, or an acceleration (a)
Force, momentum, and
acceleration are all vectors
What is the natural state of motion?
Newton’s First Law of Motion
A body in motion remains in motion and a
body at rest remains at rest unless acted
upon by an outside force.
OR
If the net force acting on an object is zero,
then there is no change in the object’s motion.
What happens when there are forces?
Newton’s Second Law of Motion
The change in a body’s
velocity due to an applied
force is in the same
direction as the force, and
is proportional to the force,
but is inversely
proportional to the body’s
mass.
F = ma
Or
F = rate of change of momentum
Because force is a vector,
forces only affect motion in the direction of the force.
Motion perpendicular to the force is unchanged.
F = ma can be rewritten to show that for a given force,
the acceleration is inversely proportional to the mass:
a=F/m
Do not confuse mass and density
Mass = amount of matter
Density = amount of matter per volume
Higher density means more matter packed
into same volume
Momentum: p = mv
Both p and v are vector quantities
Law of Conservation of Momentum
• If the net force acting on an object is
zero, then the total momentum of a
system remains constant.
Newton’s Third Law of Motion
“For every applied
force, a force of equal
size but opposite
direction arises”
or
For every action there is an
equal and opposite reaction
Newton's Laws of Motion
A body in motion remains in motion and
a body a rest remains at rest unless
acted upon by an outside force.
F = ma
(= rate of change of momentum)
For every applied force, a force of equal
size but opposite direction arises.
Major Conservation Laws
Conservation of energy
Conservation of momentum
Conservation of angular momentum
Angular Momentum
• angular momentum – the momentum involved
in spinning /circling = mass x velocity x radius
● torque – anything that can cause a change in an
object’s angular momentum (twisting force)
Conservation of Angular Momentum
• In the absence of a net torque, the total angular
momentum of a system remains constant.
Forces change momentum
Torques change angular momentum
Gravity & Orbits
A planet is always changing
its direction of motion.
Newton’s second law
therefore states that a force
must be acting on the planet.
Gravity provides this force.
Angular Momentum & Orbits
The angular momentum of
an orbiting planet is
conserved, i.e., it is always
the same.
This provides yet another
reason why planets move
fastest at perihelion and
slowest at aphelion.
The Acceleration of Gravity (a force)
As objects fall, they accelerate
(a = g = Fgrav/m).
We use the special symbol g to
represent the acceleration due
to the force of gravity.
At sea level on the Earth,
g = 9.8 m/s each second,
or g = 9.8 m/s2.
The higher you drop the ball,
the greater its velocity will be
at impact (force will be acting
on it longer).
Weight is the
force of gravity
acting upon an
object :
W = Fg = mg
Galileo demonstrated
that g is the same for all
objects, regardless of
their mass!
Is Mass the Same Thing as Weight?
●
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mass – the amount of matter in an object
weight – a measurement of the force due to
gravity acting upon an object
W = mg
F = ma
(weight)
When in free-fall, you
still have weight!
“weightless” is a misnomer
●
●
Objects do have weight in space
Free-fall often confused with
weightlessness
Tidal Forces
Because the gravitational force decreases with (distance)2, the attractive
force experienced by one object (e.g., the Earth) due to the gravitational
field of a second object (e.g., the Moon) varies with position (closest parts
attracted most strongly).
●
Now look at what happens when we measure the forces relative to the center of the Earth.
Tidal Friction
Tidal Friction
●
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●
This fight between Moon’s pull & Earth’s
rotation causes
friction.
Earth’s rotation slows down (1 sec every
50,000
yrs.)
Conservation of angular momentum causes
the Moon to move farther away from Earth.
Synchronous Rotation
●
●
●
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…is when the rotation period of a moon, planet, or star
equals its orbital period about another object.
Tidal friction on the Moon (caused by Earth) has slowed
its rotation down to a period of one month.
The Moon now rotates synchronously.
– We always see the same side of the Moon.
Tidal friction on the Moon has ceased since its tidal
bulges are always aligned with Earth.
●
Most of the large moons in the solar system
are in synchronous rotation.